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Learn the benefits and cautions of using copper for disease control on apples early in the season.

The use
of copper for disease control on apples has always been viewed from two
directions as the very positive disease-control benefits of coppers are
counterbalanced with the risk of phytotoxicity to trees, most notably through
russeting of apple fruit. Two major apple pathogens, the fire blight pathogen Erwinia amylovora and the apple scab
pathogen Venturia inaequalis, are
highly sensitive to copper, and we have no reason to believe that copper is a
material that is at risk for resistance development in either of these
pathogens. Thus, the potential benefit in using copper early for disease
control is high. The only limitation is application timing as coppers applied
after 0.25- to 0.5-inch green tip could result in phytotoxicity including fruit
russeting.

Primary
inoculum refers to the bacteria that cause the initial infections in a growing
season. After colonization of flower stigmas, bacterial populations in orchards
can skyrocket, and blossom blight infections can occur with significant losses
sure to follow. Therefore, limiting primary infection by starting control
practices early is a critical first step in a season-long control program.

The
predominant location of overwintering bacterial inoculum for fire blight is in
cankers. These cankers are initiated from shoot blight infections occurring in
the previous season and represent internal populations of the pathogen. As
temperatures warm up in a growing season, cankers begin to ooze bacteria that
can then be transmitted to and colonize flowers. Fire blight infections leading
to blossom blight are initiated during bloom. Bacterial colonization and infection
of open flowers lowers yield and initiates internal, systemic infections of
trees that can lead to rootstock blight and death of younger trees planted on
susceptible rootstocks. The occurrence of shoot blight (wilting and dieback of
actively growing shoots) is also typically higher in orchards where blossom
infections have occurred.

Besides
pruning and removal of fire blight cankers during the winter, the best method
for reducing initial inoculum populations of plant pathogenic bacteria in
orchards is to use an early application of copper to cover trees with a
"blanket" of copper. This tactic should be reserved for orchard
blocks where fire blight has occurred in one of the previous two years, or
blocks of highly-susceptible cultivars adjacent to blocks with recent fire
blight. Entire trees should be sprayed, not just alternate rows. High rates of
copper can be used (about 2.0 metallic
copper per acre), with timings immediately prior to the trees breaking
dormancy or up to about 0.5-inch green tip. Be sure that the correct rate of
copper is used and that sprayers are properly calibrated.

The goal
of this management practice is to have copper available to protect the plant
tissue from bacterial colonization over time as the tissue develops. Thus, the
copper being applied to trees at 0.5-inch green tip will actually be needed
during or immediately prior to bloom, when cankers traditionally are oozing. Because
the copper must be sprayed much earlier in the season than the dates of its
targeted effectiveness, this management strategy can be defeated by rain
amounts of 2 to 3 inches between green tip and bloom that wash the copper
residues off trees.

In
theory, any formulation of copper should be effective in disease control
(copper sulfate, cupric hydroxide, copper oxychloride, etc.) in that each
delivers what is needed for disease control, namely, free copper ions. These
copper ions are taken up by cells and cause toxicity by non-selectively
denaturing proteins in cells. Dr.
David Rosenberger of Cornell
University recently published an excellent article
on copper formulations. In the first full paragraph on page 8 of this
issue, Rosenberger gives a thorough review of the advantages of more
finely-ground copper formulations as compared to coarser-ground coppers for
this delayed dormant application for fire blight control.

An early copper application can
also serve as a first apple scab spray

You can
double-dip in disease prevention with this early-season copper application as
copper compounds, although not quite as effective as Captan or EBDCs for apple
scab control, can play a role in scab management as the first spray of the
season at 0.25- to 0.5-inch green tip. Copper, at 2 lbs. metallic equivalent
per acre, represents a separate mode of action that is not at risk of
resistance development.

The
green tip timing is absolutely critical for apple scab management. Although the
amount of tissue available for infection is small and the scab spore load is
typically small at this early timing, any infection that occurs early will
result in severe consequences later in the primary scab season. This is because
lesions initiated at green tip will be producing secondary spores (conidia) at
a timing between pink and petal fall that coincides with what is typically the
period of highest primary spore concentration from overwintering leaves. Thus,
early infections can be a killer because they compound the spore load in an
orchard, which can lead to significant fruit infection. It is true there is not
a lot of green tissue present in orchards at green tip. But, be certain that
spores of the scab fungus can find that tissue; for any spores released at this
timing, that is their primary function. And once they land on that susceptible
tissue, infection, producing a lesion and conidia becomes the primary function.

Finally, beware of phytotoxicity!

Copper
phytotoxicity can occur on apples, with the predominant copper problem on apples
being increased fruit russeting. Although phytotoxicity is a potential problem,
if used wisely, copper bactericides applied early will effectively begin the
2012 disease management season and lower primary fire blight disease inoculum
and control early apple scab.

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